This invention relates in general to orthopedic implantation of articulating joint prosthesis of the type known as intramedullary prosthesis or endoprostheses, such as intramedullary hip prostheses.
For many years surgeons have been able to replace the ball of the hip joint with a metal ball. This was done by removing the patient's ball and a part of the neck from the upper end of the femoral bone. A metal prosthesis implant, having a ball, neck, and stem, was then inserted into the medullary canal of the femur. Prior to such insertion, the more centrally positioned, softer, cancellous bone of the medullary canal had been rasped to form a bone cavity which was able to accept therein the stem of the prosthesis. For convenience, this bone cavity will be called herein a stem socket.
In the known art, the rasps used to form the stem socket were variable in shape and were not precisely correlated with the shape of the desired prosthesis. Transverse sectional dimensions of the stem, at most points along the length thereof, were substantially smaller than the corresponding sectional dimensions of the prepared stem socket. Hence, the stems were generally loosely received within their sockets. Many patients who received such a prosthesis were substantially impaired in their ability to move due to excessive pain and/or to the limited range of articulation of the joint which received the prosthesis.
To better stabilize such stems, they were first provided with transverse holes, and many years later they were provided with pores on their outer surfaces. Bone was expected to grow into the holes and/or pores of the stems. It was hoped that tissue ingrowth would improve the stability of the stems with respect to their sockets.
Because the shapes of the stems with pores were patterned after the shapes of the stems with holes, transverse sectional dimensions of the stems with pores were also, at many points along their length, appreciably undersized with respect to the corresponding sectional dimensions of their stem sockets. It was felt that such stem undersizing was necessary in order not to fracture the femoral bone during insertion of the stem into the prepared stem socket within the medullary canal. Because of such deliberate stem shape undersizing, the known stems lacked the necessary stability when inserted into their sockets and, therefore, could not properly transfer loads to the bone surrounding the stems.
Certain types of articulating joints sustain, in use, high mechanical loads, e.g., hip joints, where stresses occur in the bone which surrounds and defines the medullary canal. Due to poor stem fixations within the meduallary canals, many patients who received such undersized stems experienced limited motion due to pain.
Efforts to obtain improved stabilization for such undersized stems lead to the injection of a bone cement past into the prepared stem socket. The cement cures within and fills the void space between the external surface of the undersized stem and the adjacent tissue. Bone cement became widely used and is now generally accepted as a means for fixing a stem within its socket.
Bone cement typically includes an acrylic polymer powder which is pre-mixed with a compatible liquid acrylic monomer system to produce a doughy paste. The stem is rapidly inserted into the cement paste which then cures or polymerizes into hard cement between the stem and the bone. The hard cement is expected to anchor the stem within the stem socket. Using cement, variations in implantation procedure could be made, such as:
a. changing the manner in which the cement was injected into the prepared stem socket; PA0 b. reducing the viscosity of the cement in order to improve the inter-locking between the cement liner and the porous cancellous bone; PA0 c. lengthening the stem; PA0 d. increasing the medial to lateral dimensions of the stem; and PA0 e. eliminating the flange at the prosthesis neck so that only a small portion of the stem's medial and/or lateral outer surfaces at any given longitudinal elevation would impinge against adjacent dense cortical bone during manual insertion of the prosthesis. However, the dimensions of the remaining transverse sections of the stem along its length, above and below such elevation, were still made substantially smaller than the corresponding dimensions of transverse sections of the stem socket. This stem undersizing promoted ease of stem insertion and prevented fracture of the dense femoral cortical bone.
It will be appreciated that cement paste allows a surgeon only a very short time interval, typically 5 minutes, within which to fixate the stem within its socket. Subsequently, as the cement polymerizes and hardens, it may shrink leading to the creation of tiny gaps or voids between the cement and the stem on one hand, and/or the cement and adjoining tissue on the other hand. Such voids have been known to adversely affect the ability of the cured hard cement to uniformly transfer load stresses between the stem and the surrounding bone.
But, when the cement non-uniformly transfers such load stresses, there can result a loss of bone starting from the upper end of the femur and leading to a gradual degradation of the useful life of the implantation. In addition, the hard cement itself can be expected to fracture, even as early as 3 to 5 years after surgery. In some extreme cases, cement failure also leads to structural stem fracture. When the cement and/or the stem fractures, the patient suffers great pain, disablement, and requires a new implantation.
A re-implantation of a new prosthesis requires that the old prosthesis be forcefully removed, the hard cement drilled out, and the medullary canal re-reamed, all of which may lead to trauma and dangerous side effects in the patient's body.
Also, the possible migration of unreacted monomer from the bone cement to tissue, and the need for the bone cement to undergo an exothermic polymerization may result in serious damage to tissue surrounding the prosthesis. Such damaged tissue leads to a loosening of the stem within its socket.
Attempts to develop a cementless stem fixation involved using pores on the stem surface, as above mentioned, or adding around the stem of a porous outer coating consisting of a ceramic, polymeric, or of a composite of polymer, glass, and/or ceramic. The coated stem was inserted into the stem socket without cement, see for example U.S. Pats. Nos. 3,938,198, 3,986,212, 4,164,794 and 4,307,472.
One known type of porous composite coating, which has the ability to encourage tissue to grow into its pores, is described in "Porous Implant Sysems for Prosthesis Stabilization" by C. A. Homsy, et al, Reprint from Clinical Orthopaedics, Nos. 89, Nov.-Dec. 1972, pp. 220-235, and in U.S. Pat. No. 3,992,725 and foreign patents corresponding thereto. This known coating was bonded to the stems of conventionally-shaped and sized prostheses.
The use of conventionally-shaped prostheses was responsible for many clinical failures. It was discovered that the void space in the stem socket was surrounded substantially by relatively soft cancellous bone which could not sustain the mechanical stress loads imposed thereon. Therefore, the advantages of this known composite coating were not fully utilized until the advent of the present invention.
The success of any implant is typically measured by its ability to assume and carry out the natural functions of the joint in which it is implanted. Thus, an implant must be capable of sustaining the required compressive and flexural stresses imparted to it during normal joint movements.
Prior to this invention, known medullary prosthesis, especially hip joint prosthesis, frequently failed to accommodate normal body functions primarily because the significance and criticality of the transverse sectional dimensions of the stem were not fully appreciated.
The present invention is rooted in the recognition of the importance and criticality of the stem's transverse sectional dimensions, along the entire length thereof, relative to the corresponding transverse sectional dimensions of the medullary canal, as defined by the surrounding dense, cortical bone, known as cortex.
The primary objects of this invention are to provide an improved implantation technique utilizing a novel intramedullary prosthesis characterized by its ability to achieve (1) an adequate initial stabliization within the stem socket, (2) an enduring subsequent stem stabilization, (3) a distributed longitudinal load transfer, (4) an improved load transfer between the stem and surrounding hard cancellous bone and cortical bone, and (5) reduced localized stress zones in the bone opposite to and facing the entire stem.